The background of the present invention will be described with reference to the drawings using an example of manufacturing a stretched film. FIG. 1 is a view showing an overall schematic configuration of a general sheet manufacturing facility, and FIG. 2 is an enlarged perspective view of the main parts of a die 4 shown in FIG. 1.
The sheet manufacturing facility of FIG. 1 (1) forms a sheet material such as polymer extruded by an extruding machine 3 to a sheet form by means of a die 4 in which a thickness adjusting means 10 of the sheet is arranged in plurals in the transverse direction of the sheet; (2) further performs processing such as stretching on the formed sheet 1 by means of a stretching machine 2; (3) measures the sheet thickness as a distribution in the transverse direction of the sheet by means of a post-processing thickness gauge 8 (the thickness distribution measurement value in the transverse direction of the measured sheet is hereinafter sometimes referred to as thickness profile); and (4) winds the sheet 1. In such sheet manufacturing facility, attempt is generally made to bring the measured thickness profile closer to the target thickness profile set in advance. Thus, the thickness measurement value at the thickness measuring position corresponding to the thickness adjusting means 10 of the sheet arranged in plurals in the transverse direction of the die 4 is brought closer to the target value set in advance. Each thickness adjusting means 10 is thus controlled through a computer 14, which calculates the manipulated variable to be applied to the thickness adjusting means 10 based on the thickness profile measured in the post-processing thickness gauge 8, and a control means 9 for applying the manipulated variable to the thickness adjusting means 10. FIG. 3 is a view describing a correspondence relationship between the thickness adjusting means 10 in the die 4 and the thickness adjusting means at the position of the post-processing thickness gauge 8. The upper horizontal line of FIG. 3 shows the sheet in the die 4, and the lower horizontal line shows the sheet in the post-processing thickness gauge 8. The polymer that has passed positions 21a to 21d of each thickness adjusting means 10 in the die 4 is subjected to processing such as stretching in the transverse direction of the sheet, and passes positions 22a to 22d in the transverse direction of the sheet at the thickness measuring position. Therefore, the thickness adjusting means 10 at 21a needs to be controlled so that the thickness measurement value at 22a becomes closer to the target value.
In such sheet manufacturing method, it is important that the correspondence relationship between each thickness adjusting means 10 and the sheet thickness measuring position is accurately determined. If not accurately determined, the sheet thickness at the position different from the position to be essentially adjusted changes, for example, the sheet thickness at 22b changes if the thickness adjusting means at 21b is operated to adjust the sheet thickness at 22a in FIG. 3, whereby the thickness of the sheet cannot be accurately controlled, and the quality of the sheet lowers.
The above problem mainly results from the fact that uniform dimensional change in the transverse direction of the sheet cannot be realized in processing involving dimensional change in the transverse direction of the sheet such as stretching, foaming, or the like. In manufacturing a sheet that is non-stretched and is merely formed to a sheet form by the die 4, the correspondence relationship between each thickness adjusting means and the sheet thickness measuring position excluding the ends in the transverse direction of the sheet having a neck-in when extruding the polymer can be more or less responded by a geometric position relationship. In performing stretching process on the sheet in the transverse direction, the stretching magnification in the transverse direction of the sheet is not uniform with respect to the position in the transverse direction of the sheet due to temperature unevenness etc. during the stretching process, and thus each thickness adjusting means and the sheet thickness measuring position after stretching cannot be in a correspondence relationship with merely the geometric position relationship.
Regarding such problems, methods of patent document 1 and patent document 2 have been proposed as a method of arranging a thickness gauge of the sheet before stretching (pre-processing thickness gauge), and obtaining the correspondence relationship from the thickness profile of the sheet before stretching and the thickness profile of the sheet after stretching measured by the post-processing thickness gauge. In both methods of patent document 1 and patent document 2, the fact that the correspondence position of each thickness adjusting means more or less can be geometrically obtained in the sheet before stretching is used. The method described in patent document 1 first measures the thickness profile of the sheet before stretching, divides the thickness profile of the sheet before stretching to a constant zone, and obtains a partial cross-correlation function between the divided profile of each part of the sheet before stretching and the thickness profile of the sheet after stretching to detect the correspondence relationship between the position in the transverse direction of the sheet in the thickness gauge of before stretching and the position in the transverse direction of the sheet in the thickness gauge of after stretching. The correspondence position of each thickness adjusting means in the sheet after stretching is detected from the correspondence position in the sheet before stretching of each thickness adjusting means obtained through geometric calculation and empirical method.
In the method described in patent document 2, a reference correspondence position having a pair of correspondence relationship of the sheet before stretching and the sheet after stretching is set. The correspondence of the position in the transverse direction of the sheet in the thickness gauge of before stretching and the position in the transverse direction of the sheet in the thickness gauge of after stretching is obtained such that the mass that passes per time from the reference correspondence position is the same between each region of the sheet before stretching and each corresponding region of the sheet after stretching. The correspondence position of each thickness adjusting means in the sheet after stretching is then detected from the correspondence position in the sheet before stretching of each thickness adjusting obtained through geometric calculation and empirical method.
Through the use of such methods, the correspondence relationship of the thickness adjusting means can be estimated without generating loss during sheet manufacturing a certain situation.
[Patent document 1] Japanese Laid-Open Patent Publication No. 63-315221
[Patent document 2] Japanese Laid-Open Patent Publication No. 9-164582